An image reading apparatus such as a scanner enables a carriage to move under a fixed document to read the document. In the image reading apparatus which enables the carriage to move to read the document, due to an effect of variation of an assembly, there is a case in which an actual moving route of the carriage deviates from an intended moving route.
Further, in a case in which there is a foreign matter on a light ray from the carriage to a sensor, a dent is generated in a gain due to the foreign matter, which is detected by the sensor. If the actual moving route of the carriage deviates from the intended moving route, there is a case in which a position of the dent deviates together with movement of the carriage.
Thus, there is a problem that the image reading apparatus does not properly carry out shading correction for correcting defects caused by the foreign matter.